1. Field of the Invention
This invention relates to an apparatus for controlling ignition timing in an internal combustion engine, more specifically to an apparatus for controlling ignition timing in an internal combustion during the occurrence of knocking.
2. Description of the Prior Art
In an apparatus for controlling ignition timing in an internal combustion engine it is important to cope with knocking. When knocking occurs, it is essential to eliminate it immediately so as to protect the engine from damage. It is also necessary to ensure that knocking prevention will be effectively carried out during all operating conditions of the engine including the times when the engine is in a transient state of operation. Further, after knocking has been prevented, it is necessary to promptly restore the ignition timing to its initial value so as not to cause a reduction in engine output, but this must be done while continuing to prevent recurrence of knocking. As shown for example in Laid-open Japanese Patent Application No. 58(1983)-82074, it is well known in the prior art to eliminate knocking by retarding the ignition timing when knocking begins. In the prior art, however, the retardation of the ignition timing upon the occurrence of knocking has been carried out in increments of one degree so that the target ignition timing can be reached only over a considerable number of firings. This is particularly troublesome in the low engine speed operating region since the time interval between firings is relatively long in this region and it therefore takes a long time to eliminate knocking once it starts. As a result, the drivability of the vehicle powered by the internal combustion engine is impaired and the danger of the engine being damaged is increased.
The prior art has also entailed drawbacks as regards the control of ignition timing during transient states of engine operation, e.g. during rapid acceleration. This is true, for example, when the shape etc. of the combustion chamber is designed so as to enable the ignition to be timed near the knocking limit during normal engine operation in order to enhance engine operating efficiency. In such cases, the conventional ignition timing apparatus such as that disclosed in Laid-open Japanese Patent Application 56(1981)-20765 are incapable of completely eliminating knocking during rapid acceleration, and this is true even of those that employ knocking feedback in their ignition control systems. Typically, the conventional apparatuses will either retard the timing too little to prevent the knocking or will retard it excessively and thus reduce the engine operating efficiency to an unnecessary degree.
There have also been problems regarding the advancing of the ignition timing to restore it to near its initial value after knocking has been prevented. While as disclosed in Laid-open Japanese Patent Application No. 55(1980)-91765, it has been proposed to vary the length of an ignition advance wait period (the number of firings allowed to pass before starting to advance the timing to restore it to its initial value) with reference to the knocking occurrence frequency, the prior art apparatus merely varies the wait period depending on whether or not there were one or more occurrences of knocking within the past 32 or 128 firings and does not determine the advance of ignition angle with reference to any other engine operating conditions such as by determining it with reference to the results of a detection carried out to determine whether the engine has changed from a normal operating state to a transient operating state. As a result it is impossible to ensure optimum engine performance at all times. While during operation of the engine in the low speed region it is preferable from the point of obtaining optimum output to time the ignition at or near the point where trace knocking occurs, during high speed operation it is necessary to use a more retarded ignition timing so as to prevent the occurrence of frequent knocking from damaging the engine and shortening its service life. On the other hand, when the engine is in a transient state, particularly during rapid acceleration, while it is impossible to avoid some increase in the knocking occurrence frequency, it is still necessary to hold this frequency within certain bounds in order to protect the engine. Therefore, in order to optimize engine output and vehicle drivability, in determining the restoration (advancing) of the timing after knocking has prevented it is necessary not only to vary the reference knocking occurrence frequency in consideration of the engine operating condition but also to discriminate whether the engine is in normal operating state or in a transient operating state and on the basis of this discrimination to change the reference knocking occurrence frequency so as to hold the knocking occurrence frequency to a constant value irrespective of changes in the operating condition of the engine. The prior art techniques leave much to be desired in the respect.